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United States Patent |
6,182,468
|
Stothers
|
February 6, 2001
|
Thermodynamic separation of heavier components from natural gas
Abstract
A process for separation of particularly propane, methane or ethane from
natural gas includes providing a distillation tower arrangement for
separating the heavier components discharged at the bottom from lighter
gas discharged at the top with at least two separate towers at different
pressures such that the heavier product from the higher pressure tower is
expanded and fed to the lower pressure tower. A feed gas containing the
components under a first pressure is separated into a first proportion and
a second proportion, where neither proportion is zero. The first
proportion is fed to the tower. The second proportion is compressed to a
pressure higher than the first pressure, heat is extracted from the
compressed second proportion to effect condensation thereof, the
compressed condensed second proportion is sub-cooled, expanded to the
first pressure and supplied after expansion to the distillation tower
arrangement at a position thereon adjacent the top of the distillation
tower arrangement so as to cause cooling of the materials in the
distillation tower arrangement. The method is particularly advantageous in
a low pressure supply system in which the lighter gas discharged at the
top of the tower arrangement is supplied at a pressure less than 100 psi
and more preferably less than 75 psi.
Inventors:
|
Stothers; William R. (Calgary, CA)
|
Assignee:
|
Ultimate Process Technology (Calgary)
|
Appl. No.:
|
257352 |
Filed:
|
February 22, 1999 |
Current U.S. Class: |
62/621; 62/630 |
Intern'l Class: |
F25J 003/00 |
Field of Search: |
62/630,621,631
|
References Cited
U.S. Patent Documents
4770683 | Sep., 1988 | Stothers | 62/630.
|
5881569 | Mar., 1999 | Campbell et al. | 62/621.
|
Foreign Patent Documents |
370611 | Oct., 1989 | EP.
| |
WO 95/10011 | Apr., 1995 | WO.
| |
Primary Examiner: Doerrler; William
Attorney, Agent or Firm: Battison; Adrian D., Thrift; Murray E., Williams; Michael R.
Claims
What is claimed is:
1. A method of separating heavier components from natural gas comprising:
providing a distillation tower arrangement for separating the heavier
components discharged at the bottom of the tower arrangement from lighter
gas discharged at the top of the tower arrangement;
generating a residue gas stream from the lighter gas components for supply
at a residue gas pressure to a downstream process;
providing a feed gas containing the components under a feed pressure;
separating the feed gas into a first proportion and a second proportion,
where neither proportion is zero;
feeding the first proportion at the feed pressure to the distillation tower
arrangement at a feed position thereon between the top and bottom thereof
for separation within the distillation tower arrangement;
compressing the second proportion to an increased pressure higher than the
feed pressure, the increased pressure being selected to provide a desired
operating pressure in the distillation tower arrangement and being
different from the residue gas pressure;
extracting heat from the compressed second proportion to effect
condensation thereof;
sub-cooling the compressed condensed second proportion;
expanding the compressed condensed second proportion and supplying the
second proportion after expansion to the distillation tower arrangement at
a position thereon adjacent the top of the distillation tower arrangement
so as to cause cooling of the materials in the distillation tower
arrangement.
2. The method according to claim 1 wherein the residue gas pressure is less
than 100 psi.
3. The method according to claim 1 wherein the second proportion is
sub-cooled by cool from the lighter gas.
4. The method according to claim 3 wherein the second proportion is further
sub-cooled by a refrigerant.
5. The method according to claim 1 wherein the feed gas is dehydrated prior
to separation.
6. The method according to claim 5 wherein the feed gas is dehydrated by a
molecular sieve.
7. The method according to claim 1 wherein the first proportion is cooled
by cool from a re-boiler of the tower arrangement.
8. The method according to claim 1 wherein the tower arrangement includes
at least two separate towers at different pressures thus defining a higher
pressure tower and a lower pressure tower each arranged to separate a
lighter product at the top and a heavier product at the bottom of the
respective tower arranged such that the heavier product from the higher
pressure tower is expanded and fed to the lower pressure tower and wherein
at least a portion of the lighter gas from the top of the lower pressure
tower is added to the second proportion for processing therewith and
supply to the top of the higher pressure tower.
9. The method according to claim 8 wherein the lighter gas from the top of
the lower pressure tower is fed back to the feed gas for reprocessing such
that a portion only of the lighter gas is combined with the second
proportion.
10. The method according to claim 1 wherein the residue gas pressure is of
the order of 75 psi and wherein the residue gas is fed to a pipe line at
said pressure.
11. The method according to claim 10 wherein the lighter gas from the top
of the lower pressure tower is fed back to the feed gas for reprocessing
such that a portion only of the lighter gas is combined with the second
proportion.
12. The method according to claim 8 wherein the whole of the lighter gas
from the top of the lower pressure tower is added to the second proportion
for processing therewith and supply at a common point to the top of the
higher pressure tower.
13. The method according to claim 1 wherein the supply gas is separated
from a crude oil supply and wherein the separated heavier components are
returned to the crude oil supply as a supplement thereto.
14. The method according to claim 13 wherein the seperated lighter gas is
flared.
15. The method according to claim 13 wherein the whole of the lighter gas
from the top of the lower pressure tower is added to the second proportion
for processing therewith and supply at a common point to the top of the
higher pressure tower.
16. The method according to claim 13 wherein the lighter gas from the top
of the lower pressure tower is flared.
17. The method according to claim 13 wherein there is provided a third
tower and wherein the heavier components from the bottom of the lower
pressure tower are fed to the third tower.
18. A method of separating heavier components from natural gas comprising:
providing a distillation tower arrangement for separating the heavier
components discharged at the bottom of the tower arrangement from lighter
gas discharged at the top of the tower arrangement;
providing a feed gas containing the components for supply to the
distillation tower arrangement;
the tower arrangement including at least two separate towers at different
pressures thus defining a higher pressure tower and a lower pressure tower
each arranged to separate a lighter product at the top and a heavier
product at the bottom of the respective tower;
taking the heavier product from the bottom of the higher pressure tower
which is then expanded and fed to the lower pressure tower;
separating the feed gas into a first proportion and a second proportion,
where neither proportion is zero;
feeding the first proportion to the distillation tower arrangement at a
feed position thereon between the top and bottom thereof for separation
within the distillation tower arrangement;
adding at least a portion of the lighter gas from the top of the lower
pressure tower to the second proportion for processing therewith and
supply to the top of the higher pressure tower;
compressing said second proportion and said at least a portion of the
lighter gas from the top of the lower pressure tower to a pressure higher
than the feed pressure;
extracting heat from said second proportion and said at least a portion of
the lighter gas from the top of the lower pressure tower to effect
condensation thereof;
sub-cooling said second proportion and said at least a portion of the
lighter gas from the top of the lower pressure tower;
expanding said second proportion and said at least a portion of the lighter
gas from the top of the lower pressure tower;
and supplying said second proportion and said at least a portion of the
lighter gas from the top of the lower pressure tower after expansion to
the higher pressure tower together at a common position thereon adjacent
the top so as to cause cooling of the materials in the distillation tower
arrangement.
19. The method according to claim 18 wherein the lighter gas from the top
of the lower pressure tower is fed back to the feed gas.
20. The method according to claim 18 wherein the whole of the lighter gas
from the top of the lower pressure tower is added to the second proportion
for processing therewith and supply to the top of the higher pressure
tower.
Description
BACKGROUND OF THE INVENTION
The present invention relates to the separation of hydrocarbon gases into
components of differing boiling points. The invention relates more
specifically to a method and an apparatus especially suited for separating
propane, methane or ethane from natural gas.
The applicant's prior U.S. Pat. No. 4,770,683, issued Sep. 13, 1988,
describes a process and an apparatus for distillation of two materials of
differing boiling points. A process for distillation of two materials of
differing boiling points particularly propane, ethane or carbon dioxide
from natural gas is described in which the conventional distillation tower
is divided into a first tower at higher pressure than a conventional tower
and a second tower at lower pressure. Liquid drawn from the first is
expanded to the lower pressure through two or more stages with cool
extracted at each stage and used to cool gas withdrawn from the top of the
first tower to keep the top tray at a required temperature. Gas withdrawn
from the second tower is compressed and cooled for return to the first
tower as a reflux. The use of the cool from the expanded liquid and the
use of the two towers provides an improved thermodynamic efficiency and
avoids the use of costly turbo-expanders.
In addition, a further arrangement by the present applicant is disclosed in
PCT published application WO95/10011 of Apr. 13, 1995. This discloses an
improvement to the above patent in which efficiency is enhanced by the
provision of a third tower and an arrangement by which additional cool is
supplied to the top of the high pressure tower as a reflux.
Traditionally natural gas at less than 100 psig has been ignored for lpg
recovery. Whenever such gas is processed, it is first compressed to above
300 psig before processing. However the process of the present invention,
used for separation of various materials of close boiling points generally
uses a distillation tower arrangement.
This invention is particularly concerned with separation of heavier
components from natural gas.
Ethane recovery is similar to lpg recovery in concept except that more
energy is required for refrigeration and reflux compression. This process
also applies to situations where the low pressure gas is sold at higher
pressures but the benefits compared to other processes are much less than
that described in the first paragraph where, essentially there are no
other processes that are ever considered unless the desired residue gas
pressure for the sales pipeline is above 200 psig. The use of this
technology for the recovery of ethylene in ethylene plants, will reduce
the power requirements and capital cost of the Demethanizer portion of
these plants. The above U.S. patent of the applicant was described as
being very applicable to the separation of ethane and ethylene. That
patent could also be used for the Demethanizer in an Ethylene Plant but it
is believed that this patent will be an improvement when combined with
that patent.
This invention relates to distillation processes for the separation of
close boiling point materials. Such a process is used in the extraction of
various materials generally using a distillation tower. Examples of such
separations are:
1. Recovering ethane from natural gas
2. Recovering propane from natural gas
3. Recovering carbon dioxide from natural gas
4. Recovering helium from natural gas
5. Rejecting nitrogen from natural gas
6. Recovering ethylene in ethylene plants.
This patent has optimal advantage when utilised in conjunction with a two
tower or multi-tower process described in the above United States patent.
It may also be used to advantage with other distillation patents for
example the various arrangements described in patents held by the Ortloff
Corporation.
SUMMARY OF THE INVENTION
It is one object of the present invention to provide an improved method for
separation of residue gases from natural gas which provides improved
efficiencies particularly for processing gases where the residue gases are
supplied at low pressure.
It is one object of the present invention, therefore, to provide an
improved distillation process which obtains as good or better separation
recoveries but with an improved thermodynamic efficiency and in many cases
reduced equipment cost. It is expected that this patent will improve the
Demethanizer portion of ethylene production plants.
It is another object of the present invention to provide economical means
of recovering ethane and/or lpg from low pressure natural gas that is
otherwise ignored for liquid recovery.
According to the first aspect of the invention it is provided a method of
separating heavier components from natural gas comprising:
providing a distillation tower arrangement for separating the heavier
components discharged at the bottom of the tower arrangement from lighter
gas discharged at the top of the tower arrangement;
providing a feed gas containing the components under a first pressure
sufficient to supply the gas to the distillation tower arrangement;
separating the feed gas into a first proportion and a second proportion,
where neither proportion is zero
feeding the first proportion to the distillation tower arrangement at a
feed position thereon between the top and bottom thereof for separation
within the distillation tower arrangement;
compressing the second proportion to a pressure higher than the first
pressure, extracting heat from the compressed second proportion to effect
condensation thereof, sub-cooling the compressed condensed second
proportion, expanding the compressed condensed second proportion to the
first pressure and supplying the second proportion after expansion to the
distillation tower arrangement at a position thereon adjacent the top of
the distillation tower arrangement so as to cause cooling of the materials
in the distillation tower arrangement.
The lighter gas discharged at the top of the tower arrangement is supplied
at a pressure which is selected depending the requirement of the supply.
In many cases this is a high pressure requirement greater than 100 psig
and often of the order of 500 psig. This invention however has particular
applicability and advantage when the supply pressure is less than 100 psig
thus leading to a low operating pressure.
Preferably the second proportion is sub-cooled by cool from the lighter
gas.
Preferably the second proportion is further sub-cooled by a refrigerant.
Preferably the feed gas is dehydrated prior to separation.
Preferably the feed gas is dehydrated by a molecular sieve.
Preferably the first proportion is cooled by cool from a re-boiler of the
tower arrangement.
In one example the tower arrangement includes at least two separate towers
at different pressures such that the heavier product from the higher
pressure tower is expanded and fed to the lower pressure tower. In one
arrangement of this example, the lighter gas from the top of the lower
pressure tower is fed back to the feed gas for reprocessing. In an
alternative arrangement, the gas is flared.
Preferably the lighter gas from the top of the tower arrangement is
supplied to a low pressure pipe line system having a supply pressure of
the order of 75 psi.
Preferably the lighter gas from the top of the lower pressure tower is
added to the second proportion for processing therewith and supply to the
top of the higher pressure tower.
Preferably the supply gas is separated from a crude oil supply and wherein
the separated heavier components are returned to the crude oil as a
supplement thereto.
Preferably the separated lighter gas is flared.
According to a second aspect of the invention there is provided an
apparatus for separating heavier components from natural gas comprising:
a distillation tower arrangement for separating the heavier components
discharged at the bottom of the tower arrangement from lighter gas
discharged at the top of the tower arrangement;
a feed gas supply line for a feed gas containing the components under a
first pressure sufficient to supply the gas to the distillation tower
arrangement;
means for separating the feed gas into a first proportion and a second
proportion, where neither proportion is zero
a supply duct for feeding the first proportion to the distillation tower
arrangement at a feed position thereon between the top and bottom thereof
for separation within the distillation tower arrangement;
a compressor for compressing the second proportion to a pressure higher
than the first pressure;
means for extracting heat from the compressed second proportion to effect
condensation thereof
a heat exchanger for sub-cooling the compressed condensed second
proportion;
means for expanding the compressed condensed second proportion to the first
pressure;
and a second supply duct for supplying the second proportion after
expansion to the distillation tower arrangement at a position thereon
adjacent the top of the distillation tower arrangement so as to cause
cooling of the materials in the distillation tower arrangement.
BRIEF DESCRIPTION OF THE DRAWINGS
One embodiment of the invention will be described hereinafter in
conjunction with the accompanying drawing in which:
FIG. 1 is a schematic illustration of the elements of a first process
according to the present invention using the two tower system of the above
prior patent which is particularly but not exclusively designed for
supplying the residue gas at a low pressure.
FIG. 2 is a schematic illustration of the elements of a second process
according to the present invention using the a single tower.
FIG. 3 is a schematic illustration of the elements of a third process
according to the present invention using the two tower system of the above
prior patent and which is particularly but not exclusively designed for
returning the extracted components to a crude oil processing plant to
supplement the crude oil and for flaring the residue gases.
FIG. 4 is a sketch indicating the lowest propane plus content recovery.
FIG. 5 is a schematic illustration of the elements of a fourth process
according to the present invention.
DETAILED DESCRIPTION
Turning firstly to FIG. 1 there is shown an arrangement for separating lpg+
products from a feed of natural gas leaving a residue sales gas for sale
at low pressure that is less than 100 psig.
The arrangement provides a feed supply line 10 which feeds to an inlet
separator 11 which acts to separate gas from any incoming liquid. The
liquid can be handled in a number of different ways including supply to
the free water knock out system of the crude oil processing plant in
arrangements where such is available. Alternatively, the liquid can be
passed through a dehydrator and fed to the de-ethanizer.
The inlet gas from the inlet separator 11 is supplied to an inlet
compressor 12 having an after-cooler 13. The gas is compressed to a
sufficient pressure in the compressor 12 so that after the compressor the
gas can be dehydrated in a molecular sieve 14 and processed in the lpg
recovery plant and then has sufficient pressure for the gas entering the
sales pipeline 15.
Prior to entering the dehydrator 14 in the form of a molecular sieve, a
further liquid separator 16 is provided for recycling a liquid through a
return line 17 having a let down valve 18.
As stated above, the arrangement described herein is particularly designed
for low pressure residue gas. However if the desired pipeline pressure in
the residue gas is intended to be above 600 psig, it is preferred that
compression be added to the residue gas downstream of the recovery plant
so that the tower assembly described hereinafter can operate at
approximately 400 psig.
After the inlet gas is compressed, after-cooled and the liquids extracted
in the separator 16, the gas is dehydrated in the dehydrator 14 which is
preferably a molecular sieve as described above or can possibly be a
"Dryso".TM. process which is a tri-ethylene glycol process. In such an
arrangement a sophisticated regeneration system as shown can be provided
using extractive distillation to reduce the water content of ethylene
glycol. The extracted material from the regeneration system is returned to
the feed as indicated in the supply line 20.
Downstream of the dehydrator 14, there is provided a supply line 21 which
is divided into two supply lines 22 and 23 acting to effect a proportional
division of the feed in the supply line 21. Each line includes a flow
control valve 22A and 23A which are controlled using conventional flow
control systems well known to one skilled in the art to maintain the
required proportions depending upon the measurement of various parameters
of the process.
The process further includes a processing tower arrangement generally
indicated at 30 including a high pressure tower 31 and a low pressure
tower 32. These two towers are generally as described in the above United
States patent and the disclosure of that document is incorporated herein
by reference. The two towers each comprise a distillation tower section
for effecting separation of the components in the feed so that the high
pressure tower section 31 discharges lighter gas components at an upper
discharge 33 and heavier components at a lower discharge 34. The low
pressure tower 32 has an upper discharge 35 and a lower discharge 36. The
upper gas discharge 33 provides the residue sales gas 15 while the bottom
discharge 36 of the low pressure tower provides the heavier lpg+ product
37.
The first portion of the feed gas divided into the supply line 23 is
supplied as a feed to the lower part of the high pressure tower 31. Prior
to supply to the tower arrangement, the gas in the supply line 23 is
passed through a heat exchanger R which includes a component 38A on the
line 23 and second component 38B forming a reboiler for material at the
bottom of the low pressure tower component 32. Thus the heat exchanger R
extracts cool into the component 38A to cool the supply on the line 23 and
applies heat to the component 38B acting as a reboiler to return the
material as a side feed to the lower part of the lower pressure component
32.
The supply on the line 23 is further passed through a second heat exchanger
S having a first component 39A and a second component 39B which again acts
to extract cool for the material in the line 23 and acts as a heat supply
for a side reboiler on the lower pressure tower component 32.
Gas from the top discharge 35 of the low pressure tower 32 is returned to
the feed through a supply duct 40. Prior to return to the feed, cool is
extracted from the return gas in a further heat exchanger 41 and that cool
is applied to the feed on the line 23.
Finally a refrigerator unit 42 is used to apply external cooling to the
feed prior to injection into the high pressure tower component 31 at a
feed position 43.
The second proportion on the line 22 is passed to a compressor system 44
including a compressor 45 and a heat extractor 46. The second proportion
of the gas is compressed to a pressure in the range 500 to 1400 psig so
that it can be cooled and condensed and used for injecting into the tower
arrangement as a cooling top supply.
The prior patent and the prior published application of the present
inventor disclose the use of liquid injection at the top of the high
pressure tower for maintaining a cool temperature in the high pressure
tower. In the prior application this is termed as "reflux". However in the
present invention the compressed material includes a component of the
original supply from the feed 10 and in addition includes a component from
the discharge gas from the discharge outlet 35 of the low pressure tower.
The second proportion is thus compressed in the compressor system 44 and
cooled by the cooling arrangement 46. It is then passed through a heat
exchanger 47 which extracts cool from the residue gas and supply line 48.
Further cooling is effected in a further heat exchanger L which includes
first component 49A on the line 22 and a second component 49B extracting
cool from the product 37. Further refrigeration cooler 50 is provided
using external refrigerant. Downstream of the refrigerator 50 is provided
a further heat exchanger 51 extracting cool from the residue gas on the
supply line 48.
After the passage through the heat exchangers, the second proportion of the
feed is usually totally condensed and sub-cool is provided by the heat
exchanger 51. The second proportion of the feed is then passed through a
let down valve 52 before injection into the high pressure tower 31 at a
feed entry 53.
The compression of the second proportion only provides significant
advantages in economical recoveries. In the past, all processes considered
compressing all of the inlet gas to the high pressure before processing.
In the present invention only the proportion in the line 22 is compressed
thus avoiding the necessary power requirements for compression and also
reducing capital cost.
In some situations a phase envelope of the second proportion gas has to be
considered so that an optimum pressure is chosen which provides optimum
cool recovery by the gas/liquid and thus the most economical system. The
above optimum cool recovery is usually at a pressure that is close to the
maximum cool recovery.
Turning now to FIG. 2 there is shown substantially the same arrangement
having the same first and second proportions divided into the first and
second feed systems. In this arrangement, however, the two tower process
is replaced by a more conventional single tower process as indicated in
the single tower 55 as is well known from the processes of Ortloff.
Turning now to FIG. 3, there is shown a similar system to that of FIG. 1
which utilises the two tower process of FIG. 1 including the high pressure
tower 31 and the low pressure tower 32.
This process operates similar to that previously described and is used for
enhancing or supplementing crude oil processing from a crude oil supply
60. In this arrangement the residue gas is supplied to a flare 61 so that
it is effectively at zero pressure.
The crude oil is supplied to a separator 62 where the liquid is withdrawn
on a line 63 and supplied to a dehydrator and stabilisation system
schematically indicated at 64. This can be of the type known as a "feed
water knockout" but other processing systems can be used. From the
processing system the crude supply is indicated at 65.
The discharge gas from the top of the high pressure tower is discharged on
a line 66 and is fed to the flare 61. The discharged liquid at the bottom
of the high pressure tower is fed through a supply line 67 and a let down
valve 68 to provide a feed to the top of the low pressure tower 32.
The gas separated from the crude supply in the separator 62 is supplied to
a molecular sieve 69 for dehydration of the gas. The gas is passed through
the first heat exchanger 70 and a refrigeration unit 71. The proportional
separation is effected between the lines 72 from the refrigerator 71 so
that the first portion is supplied on the line 73 to a feed location 74 on
the high pressure tower. The second proportion is fed on a line 74 through
a let down valve 75 so that the feed is lowered in pressure to the same
pressure as the discharge 76 from the top of the low pressure tower. The
feed on the line 74 is thus added to the gas discharge from the outlet 76
and this combined flow is passed through a line 78 to an inlet 79 at the
top of the high pressure tower 31. The gas in the line 78 is passed
through a two-stage compressor including compressor components 80 and 81
and cooling components 82 and 83.
A heat exchanger R including a first component 84A and a second component
84B extracts cool from the reboiler at the bottom of the low pressure
tower 32. Further heat exchangers 85, 86, 87 and 88 act to extract cool
from the discharge gas from the discharge 66. A further heat exchanger A
includes a component 89A and a second component 89B so as to extract cool
from the gas upstream of the compressor components. A refrigerant system
90 corresponds to the refrigeration system 50 of FIG. 1. A let down valve
91 corresponds to the let down valve 52. The compressed, condensed and
sub-cooled supply is expanded back to the pressure of the high pressure
tower and is injected as a reflux-cooled supply into the top of the high
pressure tower previously described.
The discharge from the top of the high pressure tower through the line 66
is divided into two sections passing to the flare 61. One proportion
passes through the heat exchangers 85, 86, 87 and 88. A second proportion
passes through the heat exchanger 70, the heat exchanger 85 and to the
molecular sieve regeneration system generally indicated at 92. Two valves
93 and 94 let down the pressure from the pressure of the high pressure
tower to the flare pressure of approximately zero.
Again therefore in the arrangement of FIG. 3, the second proportion of the
divided supply is compressed for injection into the high pressure tower at
the cooling feed at the upper end. The second part of the feed on the line
73 is not compressed thus providing significant processing economies.
The liquid from the bottom of the low pressure tower extracted from the
otherwise waste or flare gas is returned through a line 95 as a supplement
to the feed thus enhancing the supply crude 65.
When the residue gas goes to flare, recovery of C3+ is similar in concept
to the arrangement shown in FIG. 3 for the recovery of C4+. There will be
some change in heat exchanger arrangement and the temperatures will be
much colder. Similarly, the recovery of C2+ will also be similar but
colder with a different heat exchanger arrangement. One big difference in
the arrangement for C2+ and C3+ in comparison with FIG. 3 is that these
will be produced as a separate product rather than recycling the liquid
into the inlet crude stream.
The arrangement of FIG. 3 could also be modified so that the C4+ could also
be recovered as a separate product. However normally if a separate product
is desired, recovery of C3+ is desirable also. The effects of recycling
the recovered C4+ to the inlet crude stream is to reduce the content of C3
and allows the components in the stabilised crude.
When treating gases at the low pressure similar to that of FIG. 3, there is
some incentive to mount the high pressure tower above the low pressure
tower so that there does not have to be such a large pressure drop between
the towers. This raises the suction pressure for the compressor 80 but
also raise the operating temperature of the reboiler 84B.
All of these arrangements have the advantage that the overhead from the low
pressure tower is recycled to the high pressure tower. This provides an
effective reflux supply for the high pressure tower. For example in the
case of propane recovery, the low pressure tower overhead is rich in
ethane which makes very good reflux for separating propane from natural
gas.
The flow split in the feed to the bottom of the high pressure tower does
not need to be controlled by a ratio flow control. The split stream of
dehydrated feed to the recycle compressor is controlled to maintain the
suction pressure of that compressor. Thus the compressor 80 at constant
speed will deliver a constant flow rate to the high pressure tower thus
compensating for the volume of gas exiting from the discharge 76 by taking
a portion of the feed on the line 72. This is also has the effect that
when the plant is turned down in flow rate or composition, the percentage
recovery of liquid product will increase.
The power requirement for the Feed Compressor is minimized since the gas is
only compressed as much as required considering pressure drops in the
dehydrator and lpg recovery process. When the desired Residue Gas pressure
is the same or less than the Feed Pressure, very little Feed Compression
is required, resulting in much less power requirement for this process
than any other process.
Typical propane recoveries from this process are 90% using the two towers
as shown in FIG. 1. When this process is configured with a three tower
process (our United States patents 1988 and 1997 patents), 95% propane
recoveries can be easily achieved.
In addition to saving energy, the lower power requirement results in a
smaller compressor installation and a reduced capital cost compared to
other processes.
FIG. 4 is a sketch indicating the lowest propane plus content recovery.
Note that processes using the Reflux Compressor can recover lpg from much
lower Feed Pressures as long as the lpg concentration in the gas is high
enough. The Reflux Compressor adds a considerable number of applications
for economical lpg recovery compared to other processes that do not have a
Reflux Compressor.
Turning now to FIG. 5 there is shown a further modified arrangement in
which there is a three tower system including towers 100, 101 and 102. In
this arrangement the feed is again split to provide a proportional flow at
the location 103 and a portion of the feed is compressed through the
system 104 as previously described and fed at the reflux location 105 to
the tower 100. In this arrangement the gas from the top of the second
tower 101 is sent to flare 106. Also in this arrangement the gas from the
top of the tower 100 is sent to flair 7. In this arrangement there is
provided a water separation at a condenser 108 which is located upstream
of a dehydrator 109.
Note that the dehydrator 109 is located after 90.degree. of the water has
been condensed out of the gas at 38 F. The refrigerant temperature in the
chiller is 33 F so there is no danger of freezing and the chiller assures
a maximum temperature into the dehydrator. Location of the dehydrator
after most of the water has been removed greatly reduces it's size and
regeneration heat requirement. Note also that the overhead from the
de-ethanizer is not recycled, it is sent to flare along with the other
residue gas from the process. Heat for the de-ethanizer reboiler is
obtained from the process as we normally do in our other designs. This
means that heating medium is not required for this tower, but it is
required for the debutanizer. There may be situations where having this
extra tower is not warranted, trays could be added below the bottom feed
on the gas fractionator and the reboiler added to that tower. That would
be the conventional Ortloff patent.
All the metallurgy is carbon steel except for the top feed to the gas
fractionator. For this design, it would likely be wise to have the top
reflux meet the gas fractionator overhead in a small stainless vessel
having one or two trays. This stainless steel vessel would be mounted on
top of the main gas fractionator column which would have a -50 F design
temperature so could be Charpy-tested carbon steel.
Since various modifications can be made in my invention as herein above
described, and many apparently widely different embodiments of same made
within the spirit and scope of the claims without departing from such
spirit and scope, it is intended that all matter contained in the
accompanying specification shall be interpreted as illustrative only and
not in a limiting sense.
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